Even when the US Army backed out of a commitment to assume responsibility for the programme in 2003, DARPA stood by the A160 and rescued it from an early death.

The A160 programme is 10 years old and, in a series of test flights from April to May earlier this year, has finally demonstrated the eye-popping endurance and hover capability promised by designer Abe Karem since 1998.

On 15 May, the team demonstrated it could meet DARPA's threshold requirement for 18h endurance, by carrying a 135kg (300lb) internal payload for 18.7h and landing with 2h of fuel remaining.

Having thus achieved the initial goals of the demonstration programme, DARPA is now making it clear that its role is about to change.

The agency wants to transfer responsibility for the A160's transition from development into production to the manufacturer.

Interestingly, the A160's path through development is so far not unlike that of the Karem's earlier invention - the Predator unmanned air vehicle. DARPA guided both programmes through several years of development.

Demand for the Predator's capability was first registered in the Balkan wars and then exploded after the overthrow of the Taliban in Afghanistan in late 2001. At the time, the air force took over the Predator programme and has not looked back.

Stub wings

But DARPA is not completely abandoning the Hummingbird to its fate. For the next 12 to 18 months, the agency is funding a short, "bridge phase" flight-test programme that will expand the envelope and allow for tests of stub wings intended for carrying weapons.

DARPA also will support the A160 by flying it as a testbed for a variety of advanced sensors, to include the Forester foliage-penetrating radar.

DARPA is providing bridge funding for tests of stub wings

Both follow-on deals are intended, Hunt says, to "give some time for Boeing to pick up the reins". Boeing's challenge will be finding a US government sponsor to complete development and launch production, despite the lack of any funded requirement for such an aircraft in the visible future.

"We think [the A160T] provides range and endurance very similar to some fixed-wing UAVs, but the advantage is in being able to provide persistent stare in urban environments and mountainous terrain and being able to launch and recover from unprepared sites," says Grady Eakin, Boeing business development director advanced rotorcraft, on the A160.

Eakin's obvious reference to the fixed-wing Predator UAV could be revealing. Boeing may seek to position the A160 as an alternative to helicopter UAVs, such as the Northrop Grumman RQ-8 Fire Scout, and medium-altitude fixed-wing UAVs, such as the Predator.

DARPA's Hunt views the A160 as complementary to the Fire Scout rather than a competitor.

That contrasts with the programme's history, as the army selected the Fire Scout over the Hummingbird in 2003 to fulfil the Class IV UAV requirement in the Future Combat System family of air and ground vehicles.

In 2003, the A160's development was not sufficiently mature for the army's decision-makers, who were seeking to deploy new technologies immediately.

Instead, the army and DARPA agreed to fund a joint, five-year demonstration programme to allow the A160 more time to demonstrate its performance promises.

Of the 12 A160Ts being produced, four are for DARPA and the remaining eight for Navair. The latter will be used by US Special Operations Command.

Eakin says the navy has a requirement not met by the RQ-8B and its various manned helicopters. He says the service needs a mix of manned helicopters, fixed-wing UAVs and rotary-wing UAVs.

Two-speed transmission

The A160T is equipped with a two-speed transmission gearbox to enable the rotor speed to be optimised for various combinations of operating weight and altitude. "Later this year or early next year we'll do full in-flight shifting in Victorville," Eakin says.

All A160T flights to date have been conducted with the transmission in low gear, but the engine has been shifted to high gear on the ground. "It's all-composite, so it's a very light, very stiff fuselage," Eakin adds.

The rotor speed on a typical helicopter can be varied around 95-102%. "It's also got very light, very stiff blades," Eakin says. "The combination of that allows us to optimise the rotor speed and slow it all the way down to 50%.

That gives you great endurance, and also gets a little quieter. If you try to slow the rotor speed down on a standard helicopter it doesn't work, so any optimum speed rotor technologies need to be designed in from the ground up," he says.

The team is also looking at the possibility of offering a folding-blade option for the A160T.

Due to its low cross section, "when it turns nose-on it all but disappears at one mile away". When the A160T is half a mile away at 2,000ft (610m) altitude "you won't hear it".

The A160T can be flown autonomously, semi-autonomously or manually from a trailer-sized or mobile Humvee-sized ground station, or via the internet using a laptop.

"We actually passed control from a trailer here [in Victorville], to someone on the internet up in Seattle, and they've commanded the vehicle, they've controlled the payload and told it to go and fly in a particular racetrack [pattern]," Eakin says.

Boeing's unmanned helicopter development began a number of years ago with the Robinson R22-based Maverick, which used a similar ground control station, and was used to train operators. "If you're trained as a Maverick operator, you're also trained as an A160 operator," Eakin says.

Little wasted space

The maximum operating weight of the A160T is 2,950kg. "We are able to densely pack the vehicle with engine fuel or payload. There is very little wasted space in this vehicle. About half of the weight of the vehicle is in fuel," Eakin says.

The A160T weighs about 1.045kg "dry" and can carry 1,045kg of fuel, leaving a payload capability of up to 860kg. The 10.6m-long helicopter has a maximum speed of 140kt (260km/h) and a range exceeding 4,150km (2,250nm).

"We can carry well in excess of 1,000lb of payload. We've got a 20,000ft ceiling and that is limited by the Pratt engine. The aircraft is actually capable of 30,000ft and as customers would like to fly higher we may look into flying higher. We can believe we can get 170kt out of the aircraft. It goes a long way and carries a lot," Eakin says.

The A160 can be fitted with an external payload module for delivering supplies, evacuating personnel or returning hardware from the front line.

On the flight line currently are two six-cylinder A160s, plus one modified to look like a turboshaft-powered vehicle, but which is actually also powered by a six-cylinder engine, and five turboshaft powered A160Ts.

The PW207D is "a very powerful, well-packaged engine. At one time DARPA wanted to produce diesel versions of the A160 that were predicted to achieve about 40h of endurance," Eakin says.

He adds: "At the end of 2005 it was determined that the diesel engine was not going to be ready in time for the A160 and it was basically put on the side for the programme, and the rest of the programme was focused in on the turboshaft, which is a very capable helicopter engine.

If the diesel engine was ready today I think that would be revisited with gusto. The diesel promised us twice the endurance."

Boeing built four four-cylinder engine-powered examples, one of which was later converted into a six-cylinder.

Three six-cylinder A160s were then constructed, and a total of 12 turboshaft-powered UAVs will be built, including the five now on the flight line.

Automotive Engines

"Then we went through a series of early versions of the A160 using automotive engines - Subaru four-cylinder and six-cylinder - and then transitioned to turboshaft," Eakin says.

Of the remaining turboshaft vehicles, "we plan on completing most of those this calendar year".

The A160's retractable gear enables it to carry the 5.8m-long, rotating Forester UHF foliage-penetrating radar, which is a "hand in glove" design for the Hummingbird.

Following the crash of a Hummingbird in flight testing, an upgraded flight-control system will be tested later this year. "We'll have a full dual-redundant system probably early next year," Eakin says.

Regarding possible US government approval for exports of the A160T, "we're currently starting a very lengthy process to market this internationally", he adds.